1. Field of the Invention
The present invention relates to a fuel vapor processing apparatus wherein fuel vapor generated in a fuel tank is trapped or collected by a canister and the trapped fuel vapor is purged to an intake passage of an engine. More particularly, the present invention relates to a failure diagnostic system for checking a failure in air-tightness of a fuel vapor processing apparatus.
The present invention also relates to a canister which is included as one component in a fuel vapor processing apparatus and which incorporates in a casing, an adsorbent for adsorbing fuel vapor.
The present invention further relates to a pump module to be used in diagnosing a failure of a fuel vapor processing apparatus and more particularly to a pump module which permits failure diagnosis with a high accuracy.
2. Description of Related Art
As one of systems mounted on a vehicle, a fuel vapor processing apparatus has heretofore been known wherein fuel vapor generated in a fuel tank is treated without release to the atmosphere. This system is provided with a canister for trapping fuel vapor. Fuel vapor is trapped by once adsorbing the vapor on an adsorbent present in the interior of the canister. By utilizing a negative intake pressure developed in an intake passage during operation of an engine, a fuel component contained in the fuel vapor trapped by the canister is purged to the intake passage through a purge passage and is thereby used for combustion in the engine.
In this type of a treatment system, if the flow path from the fuel tank to the intake passage should be perforated in a certain position for some reason or other or if a sealing defect should occur in a pipe joint portion, there is a fear that fuel vapor may leak to the outside, and it is impossible to treat the fuel vapor properly. To cope with such a leakage trouble; it is necessary to perform diagnosis in an early stage. A related failure diagnostic system is described for example in Japanese unexamined patent publication No. 2003-155958 (pages 2 to 7, FIGS. 1 to 5).
The diagnostic system described in the above publication has, separately from a canister, a pump module for pressurizing the interior of the canister. The pump module sends out air into the canister, whereby a flow path extending from a fuel tank to an intake passage is pressurized. In this diagnostic system, when the internal pressure of the flow path once applied becomes lower than a predetermined value, it is determined that there is a leakage trouble in a fuel vapor processing apparatus. The pump module includes a pump, a solenoid valve and a filter, which components are integrally accommodated within a housing.
The above pump module can be mentioned as one of components having a failure diagnosing function necessary for the fuel vapor processing apparatus. The pump module is usually disposed near the canister. Generally, however, the space for mounting various components in a vehicle is limited. In view of this point, a concrete mounting method may be attaching the pump module integrally to the canister and fixing this integral combination to a vehicular frame. For example, in Japanese unexamined patent publication No. 2003-113745 (pages 2 to 6, FIGS. 1 to 3) there is described a structure wherein a valve is attached to an upper portion of a canister through first and second connecting parts. Adoption of the mounting structure described in this publication No. 2003-113745 may be effective also for the above canister and pump module.
On the other hand, another failure diagnostic system is disclosed in Japanese unexamined patent publication No. 2002-4959 (pages 2 to 3, FIG. 1). A basic way of thinking of this failure diagnostic system is as follows. An orifice is provided and a pressure change (leakage quantity) as a reference value is measured in advance. Thereafter, a flow path in a fuel vapor processing apparatus is made into a hermetically sealed space and is then brought into a state of a negative pressure. Subsequently, a pressure change in the flow path is compared with the pressure change as a reference value to determine whether fuel vapor is leaking or not.
However, in the failure diagnostic system disclosed in Japanese unexamined patent publication No. 2003-155958, the interior of the flow path is pressurized, so if the flow path should be perforated, the application of pressure may rather cause leakage of fuel vapor to the outside through the hole. Contrary to the application of pressure, it may be effective to once reduce the internal pressure of the flow path and determine that fuel vapor is leaking when the internal pressure of the flow path once reduced has become higher than a predetermined value. According to this method using such a reduced pressure, even if the flow path should be perforated, there is no fear of leakage of fuel vapor to the outside unlike the foregoing pressurizing method.
However, even if the reduced pressure method is adopted, a large amount of fuel vapor is developed in the fuel tank when the temperature is high. Accordingly, even if the pressure is reduced in this state, the internal pressure of the flow path is difficult to become negative and it takes much time for failure diagnosis. A solution to this problem may be the omission of failure diagnosis when the temperature is high. But, if diagnosis is not performed at a high temperature, the diagnosis is less frequently performed accordingly. This is contrary to the demand for early diagnosis upon occurrence of a failure or trouble.
On the other hand, in the diagnostic system disclosed in Japanese unexamined patent publication No. 2003-155958, if the pump module is connected to an upper portion of the canister, not only the size of the entire canister becomes larger, but also the center of gravity of the entire canister is largely offset to the outside by the pump module. In this state, for example, if the canister is attached to a vehicular frame through a bracket, the pump module will undergo vibration during vehicular running and vibrate, with consequent fear of breaking in the connection between the canister and the pump module. Thus, there has been a problem in point of reliability as a diagnostic system.
According to the diagnostic system disclosed in Japanese unexamined patent publication No. 2002-4959, in a certain mounted state thereof on a vehicle, there occurs a case where an orifice is positioned lower than the other flow path portion. If the orifice is thus positioned low, there is a fear of the orifice being plugged up with dust, water, fine adsorbent powder, intra-pipe oil, plastic extract, or assembling oil. Once the orifice is thus plugged up, it is no longer possible to effect diagnosis with a high accuracy.